The present invention relates to a test method for evaluating reliability of a ferroelectric memory device that includes a capacitor having a capacitor insulating film of a ferroelectric material.
Recent rapid growth of digital technologies has been accelerating the trend toward high-speed processing and storage of larger volumes of information. Therefore, higher integration and higher performance have been demanded for semiconductor memory devices which are used in electronic equipments. In response to such demands, ferroelectric memory devices that use ferroelectric capacitors having spontaneous polarization characteristics as capacitors of semiconductor memory devices have been actively studied and developed. Ferroelectric capacitors included in ferroelectric memory devices have a property that the polarization direction of ferroelectric capacitors is easily inverted by application of a relatively low external voltage. Ferroelectric memory devices are memory devices which store a certain polarization direction as “0” and the opposite polarization direction as “1” by using this ferroelectric capacitors' property. The ferroelectric memory devices are characterized by high-speed write/read operation with a voltage that is lower than that in the conventional examples. Moreover, polarization of ferroelectric capacitors is held as residual polarization which does not disappear even when the external voltage is removed. Non-volatile memories capable of retaining stored data for a long time even in the power-off state can be implemented by using this ferroelectric capacitors' property.
In general, some characteristic tests in view of physical properties of ferroelectric capacitors are conducted in the reliability test for such ferroelectric memory devices. One of the characteristic tests is a retention characteristic test in view of depolarization characteristics of ferroelectric capacitors.
When a ferroelectric capacitor retaining polarization of one direction is stored, the retained polarization reduces with time. Such characteristics of ferroelectric capacitors are called depolarization. The depolarization characteristics affect data retention characteristics (retention characteristics) of ferroelectric memory devices. The retention characteristics are characteristics of whether or not data can be read from a ferroelectric memory device after the ferroelectric memory device has retained data of one direction for a long time. When depolarization of a ferroelectric capacitor is extremely large, the data cannot be retained, and therefore, cannot be read from the ferroelectric memory device (read failure).
In view of the above characteristics of ferroelectric capacitors, the retention characteristic test of a ferroelectric memory device examines whether or not data can be read from a ferroelectric memory device after the ferroelectric memory device has retained data of one direction for a period corresponding to the data retention guarantee time. The retention characteristic test is the most basic reliability test for evaluating capability of a ferroelectric memory device as a non-volatile memory, and is widely used in various applications.
An acceleration test is used to carry out the reliability test within a limited period of time. In the acceleration test, reliability is evaluated by using more stressed conditions than actual use conditions. Data retention guarantee time of ferroelectric memory devices is usually 10 years. In order to carry out the retention characteristic test of a ferroelectric memory device within a short period of time, the acceleration test uses excessively stressed conditions and evaluates the retention characteristics of ferroelectric memory devices after the ferroelectric memory devices has retained data for a period corresponding to 10 years. A temperature stress is commonly used in the acceleration test. In other words, an excess temperature is applied to the ferroelectric memory devices.
In the acceleration test using a temperature stress (hereinafter, referred to as “temperature acceleration test”), the test corresponding to guarantee time t1 required for a guarantee temperature T1 is conducted by using an acceleration temperature T2 higher than the guarantee temperature T1 in order to reduce the time required for the test from guarantee time t1 to test time t2. In order to conduct the temperature acceleration test, it is required to determine in advance the test time t2 for the acceleration temperature T2 corresponding to the guarantee time t1 for the guarantee temperature T1. The relation between the guarantee time t1 and the test time t2 varies depending on temperature dependence of reliability characteristics to be tested. Therefore, the test time t2 is commonly determined by selecting a parameter relating to the reliability characteristics and measuring temperature dependence of the selected parameter.
For example, the time a ferroelectric memory device becomes defective due to the reliability characteristics to be tested, that is, the “life” of a ferroelectric memory device, is selected as a parameter, and the test time t2 is determined based on temperature dependence of the life. However, in the case where the life of the ferroelectric memory device is used as a parameter, no information about temperature dependence of the life can be obtained until the ferroelectric memory device's life actually expires. Therefore, it often takes a lot of time to determine the test time.
As for ferroelectric memory devices, “retained polarization,” polarization that is retained in a ferroelectric capacitor when data is written to a ferroelectric memory device, is commonly selected as a parameter, and the test time is determined based on temperature dependence of change of retained polarization with time (e.g., Japanese Laid-Open Patent Publication No. 11-102600). Change of retained polarization with time can be obtained from right after the measurement is started, regardless of the life of the ferroelectric memory device. Accordingly, the test time can be determined in a relatively short time based on temperature dependence of the change of retained polarization with time.
Hereinafter, a conventional method for testing retention characteristics of a ferroelectric memory device will be described with reference to the accompanying drawings.
FIG. 23 is a flowchart of the conventional method for testing retention characteristics of a ferroelectric memory device.
In a data writing step S10, data of one direction is written to a ferroelectric memory device.
In a high-temperature storing step S20, the ferroelectric memory device is stored at an acceleration temperature T2 for prescribed test time t2. The acceleration temperature T2 is higher than a guarantee temperature T1.
In a test-time determining step S30, the prescribed test time t2 is determined based on temperature dependence of change of retained polarization with time. The retained polarization is polarization that is retained in a ferroelectric capacitor when the data is written to the ferroelectric memory device. The test-time determining step S30 is conducted separately.
Whether the data retained in the ferroelectric memory device can be read or not is examined in a reading examining step S40 and the test is completed.
The test-time determining step S30 will now be described in detail.
As described above, in the temperature acceleration test, the test time is determined by selecting a specific parameter relating to reliability characteristics to be tested and measuring temperature dependence of the selected parameter. As mentioned earlier, retention characteristics of a ferroelectric memory device are affected by depolarization characteristics of ferroelectric capacitors of the ferroelectric memory device (characteristics that polarization retained in a ferroelectric capacitor decreases with time). Therefore, in the test-time determining step S30, retained polarization is selected as a parameter, and the test time t2 is determined based on temperature dependence of change of retained polarization with time.
For example, as shown in FIG. 24, change of retained polarization PSS with time is first obtained for both the guarantee temperature T1 and the acceleration temperature T2. Retained polarization PSS′ after the guarantee time t1 at the guarantee temperature T1 is then obtained. Thereafter, the time the retained polarization PSS of the acceleration temperature T2 becomes equal to the value PSS′ is obtained. The time thus obtained is determined as test time t2. By using the test time t2 and the acceleration temperature T2, the condition after the guarantee time t1 at the guarantee temperature T1 can be reproduced in a short period of time. As a result, the temperature acceleration test of retention characteristics can be conducted.
When the data can be read from the ferroelectric memory device in the reading examining step S40, that is, when read failure does not occur in the reading examining step S40, it can be evaluated that retention characteristics of the ferroelectric memory device can be guaranteed for at least guarantee time t, at the guarantee temperature T1.